Add More Oscillators per Note

[essenbee]

Add 2 maybe 3 oscillators that can be mixed into a note, with settings for:

• [x] Level (dB)
• [x] Wave Type (Sine, SawTooth, Square, Triangle, White Noise, Pink Noise)
• [x] Relative tuning

[essenbee]

Added 2 more voices per note, with GUI controls for Level, ADSR Envelope, relative tuning (octaves and semitones), and Wave Form. Still need to add fine tuning in cents, and also allow negative relative tuning compared to the Main Voice. Do we need individual low pass filters?

[essenbee]

Negative relative tuning for Octaves and Semitones implemented.